Artificial Light at Night: State of the Science 2022
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Artificial Light at Night:
State of the Science 2022
International Dark-Sky Association
This briefing summarizes the current state of knowledge about objects and the background sky, making it difficult to observe
how the widespread and growing use of artificial light at night them.
interacts with six key topics: the night sky (Section 1); wildlife A slow but steady rise in skyglow in much of the world
and ecology (Section 2); human health (Section 3); public safety leads to gradually degraded visibility of the natural night sky
(Section 4); energy security and climate change (Section 5); and and a transformation of lighted outdoor spaces. Such a situ-
social justice (Section 6). It also includes a discussion of the
ation, changing slowly over decades, may go unnoticed due
emerging threat from light pollution caused by objects orbit-
to a psychological effect know as a “shifting baseline” (9).
ing the Earth (Section 7). Finally, it concludes with a discussion
of the knowledge gaps that exist within these topics and the re- This applies to various aspects of artificial light on a ‘nor-
search questions whose answers can fill the gaps (Section 8). It is mal’ night: the number of visible stars, the amount of artifi-
intended be useful to those seeking to broaden their understand- cial light associated with perceptions of safety, and the expe-
ing of research on the causes and consequences of artificial light rience of using non-visual senses such as hearing and balance
at night. at night. Along with other effects, the loss of the night sky is
barely noticed.
Introduction
Light pollution is surging in both its presence and reach
across our planet (1, 2). It is the source of both known and
suspected harm to the nighttime environment (3). Scientific
studies suggest the over-use of artificial light at night (hence-
forth, ‘ALAN’) is the main source of light pollution (4, 5).
The main challenge they identify is how to maximize the hu-
man benefits of ALAN while limiting its potentially negative
social and environmental impacts (6–8).
1 The Night Sky Figure 1. The streetlight at left emits light in many different directions. Some of
the light rays (1) travel upward into the sky and pass completely through Earth’s
atmosphere. Satellites detect some of these rays (2) as they pass over the nighttime
Light emitted into the night sky makes it difficult to see the side of our planet. In other cases (3), the atmosphere scatters rays back to the
stars. On the ground, ALAN makes the nighttime environ- ground. This light becomes the familiar “skyglow” seen over cities. Some of the
rays traveling downward (4) reflect off the ground into the sky where they are seen
ment brighter. Weather changes like clouds and snow on the by satellites. Lastly, some rays scatter into astronomers’ telescopes (5), blocking
ground can make this impact worse. New and inexpensive their view of the universe. Credit: IDA.
light sources like white light-emitting diodes (LEDs) have a
growing impact on both the night sky and outdoor spaces at Remote sensing of light pollution
night.
“Remote sensing” is a method of measuring the properties
The most immediate symptom of light pollution is the of something at a distance without directly sampling it. It is
phenomenon of “skyglow”. It brightens the night sky in and often applied to observations of our planet made by orbiting
near cities where large installations of outdoor lighting ex- satellites. When those satellites look at light on the night
ist. The lower layers of the Earth’s atmosphere scatter light side of Earth they provide a view of the global scale of the
emitted near the ground. Some of that light escapes the at- problem of light pollution (1, 10, 11).
mosphere where Earth-orbiting satellites detect it, but many Figure 2 shows a global map of night lights made with
light rays encounter molecules and small particles in the at- remote sensing observations (12). This is a composite image
mosphere. These interactions redirect the paths of some of composed of observations of Earth made over many nights in
the light rays back down to the ground. Observers there see one year. It gives the appearance of our planet as if it were
light appearing to come from the night sky itself; see Fig- simultaneously night everywhere at once. It also ensures that
ure 1. Skyglow competes with the faint light of astronomical the result does not include clouds or light from the aurora
objects in the night sky. It lowers the contrast between those near the Earth’s poles.
1
The camera used to make this map uses a sensitive de- The rise of solid-state lighting may threaten dark skies
tector that records faint light in the visible spectrum. It can Global light pollution has increased in recent years in part be-
resolve features on Earth smaller than one kilometer in size. cause of the introduction of solid-state lighting (SSL). This
This is smaller than the size of most cities, so the images give kind of lighting uses semiconductor materials to generate
detailed information about the number and characteristics of light. It differs from earlier technologies that used electric
various light sources on the ground. Images like these dating currents in tubes of gases like sodium vapor. Those earlier
from as early as the 1970s are available to the public and for methods of making light once dominated the global outdoor
scientific study. lighting market.
In recent years, researchers have learned much about the The most familiar kind of SSL technology is the white
spread of light pollution across the globe by studying remote LED. This technology now accounts for almost 50% of
sensing data. They found that skyglow fouls the night sky for global lighting sales (28). The lighting market’s explosive
more than 80% of all people and more than 99% of the U.S. growth in recent years is due in part to the exceptional en-
and European populations (10). ergy efficiency of SSL, which is up to ten times higher than
Both the amount of artificial light seen on Earth at night earlier technologies like incandescent filament lamps. While
and the land area that light covers grow by about two per- one-for-one SSL replacements save energy compared to ear-
cent each year on average.(Figure 3) (1). Yet, both numbers lier technologies (with beneficial impacts; see Section 5), the
vary across our planet (13). There are only a few countries in energy efficiency and low cost of SSL can encourage over-
which they seem to be either stable or decreasing (1, 14). lighting (with negative impacts; see Sections 2, 3, and 5). In
Satellite remote sensing used to make studies like these is order achieve the full promise of SSL, factors such as the
not perfect. For example, the best available satellite cameras spectrum and distribution of the light source should be care-
are not sensitive to some colors of light. In particular, they fully designed.
do not see the blue light emitted by white LED lighting. This The rapid rush to adopt and install SSL has changed the
means that key light pollution indicators are probably under- color of artificial light emitted into the nighttime environ-
estimated. Combining satellite data with ground-based ob- ment (29, 30). White LED lighting generally emits much
servations can improve the reliability of results (15), but the more short-wavelength (i.e., blue) light than other technolo-
need for new, dedicated orbital facilities to address important gies. This causes a shift in the color of cities as they transition
research questions is urgent (16, 17). This is especially true to SSL (31). It may also make skyglow over cities worse even
given that some Earth-observing satellite missions, such as when the number of lumens – that is, the amount of light to
NASA’s Terra, are slated to end in coming years. which the human eye is sensitive – used is the same (32–34).
This may extend the impact of city lights much farther into
Environmental conditions change night sky quality adjacent, ecologically sensitive areas (35, 36). It also specifi-
Cloudy conditions tend to make skyglow more intense in cally threatens the productivity of ground-based astronomical
urban and suburban areas because overcast nights can in- observatories (37), which rely on sites with dark night skies
crease the intensity of light reflected back down to ground in order to produce new knowledge about our universe. How-
level by up to ten times (18, 19). However in rural areas ever, the characteristics of LED lighting can enable its more
with few light sources, cloud cover tends to darken the night efficient use, often requiring less light for the same applica-
sky (20). This is because clouds efficiently absorb and scat- tions than previous technologies (38). When cities plan LED
ter light from both natural and artificial sources, decreasing retrofits carefully, they can hold light pollution steady or even
the amount reaching the ground. Skyglow is also sensitive to reduce it (39–41).
very small particles in the air (21), and it can be increased by
air pollution (22). Dark-sky conservation and astrotourism
Ice and snow make skyglow worse because they re- Meanwhile, the ongoing conversion of world outdoor light-
flect much more light than darker ground covers. This en- ing to SSL, and its potential to increase skyglow, may work
hances the apparent nighttime artificial light emissions from against dark-sky landscape conservation goals. Public inter-
cities (23). Snow cover on the ground under clear-sky con- est in visiting naturally dark places is increasing (42). This
ditions can increase night sky brightness by up to three has created a new kind of “astrotourism” (43, 44) with sig-
times (24). When clouds cover the sky in the winter months, nificant revenue-generating potential (45). This may in turn
light reflected from both snow and clouds amplifies sky- encourage lighting practices and public policies that protect
glow. The result can raise the night sky brightness by over night skies, yet it calls into question what defines a “dark
3,500 times compared to overcast conditions with no artifi- sky” (46) and how it should be quantified (47, 48). It also
cial light (25). Even in clear weather, the tendency of ground requires understanding how to measure or describe nighttime
covers like asphalt and concrete to reflect light can raise night darkness to best preserve it (49, 50). Limited evidence sug-
sky brightness (26, 27). gests that efforts that recognize the value of dark skies and
support their conservation may have positive benefits in re-
IDA | State of the Science 2022 | 2
Figure 2. A cloud-free composite image of the Earth at night made using Earth-orbiting satellite data for the year 2016. Credit: NASA Earth Observatory/Goddard Space
Flight Center/J. Stevens/M. Román.
Figure 3. This figure from reference (1) shows how nighttime lights on Earth changed during 2012-2016. The map on the left shows the change in the land area showing
indications of artificial light as seen from space, and the map on the right shows how much the brightness of the light changed. Red colors mean increases in lit area and/or
brightness during the study period and blue colors mean decreases. Yellow areas were unchanged.
ducing skyglow on regional scales (51). natural levels of light that vary by factors of over one bil-
lion times (Figure 4). The rising and setting of the Sun and
moon set light levels and the timing and duration of light ex-
2 Ecological Impacts posure. They are the most important sources of light in the
natural environment, and they establish cues that species look
ALAN exposure impacts almost every species studied by sci-
for around them. This tells them when to engage in certain
entists. It interferes with their biology and changes how they
behaviors like finding food and mates.
interact with the environment. This harms ecosystems and
Some species rely on very dim sources of natural light,
can make plants and animals less resilient in the face of en-
such as starlight, for orientation and navigation (53–57). Ar-
vironmental change.
tificial light can disrupt the activities of these species. Their
Organisms at or near the surface of the Earth experience
IDA | State of the Science 2022 | 3
change (104–106).
Artificial light exposure seems to weaken the immune
systems of some organisms (107–109). Parents may pass that
weakness to their offspring (110, 111). Light at night expo-
sure may thus leave some species more vulnerable to both
predators and parasites (112, 113). Researchers also find that
light exposure often occurs alongside noise caused by human
activity (114). The combination of artificial light and acous-
tic noise can further harm some species (115, 116).
How light affects biology
Light has two kinds of effects on plants and animals: inter-
nal (through physiology) and external (through interactions
with the environment and with other species). Physiologi-
Figure 4. Natural illumination during the day and at night. The solid black line is the cal effects of ALAN exposure include disruption of normal
amount of light falling on surfaces near the ground. Certain times are indicated: SS chemical signaling in organisms (117, 118). This signaling
= sunset (when the Sun’s angle above the horizon reaches 0◦ ); CT = civil twilight relates to the circadian rhythm, a roughly 24-hour cycle of
(Sun angle = −12◦ ); NT = nautical twilight (Sun angle = −15◦ ); AT = astronomical
twilight (Sun angle = −18◦ ). Note that the increments on the vertical axis increase activity tied to the length of the day. Exposure to sunlight,
in powers of ten. The horizontal axis shows the angle above or below the horizon followed by many hours of darkness, establishes an environ-
of the moon. Dotted lines show the the illumination by the moon for its full and mental cue. This helps ‘entrain’ the circadian rhythm when
quarter phases. Cloud cover decreases the ground brightness by the amount in the
shaded region at upper left. The shaded region at lower right is the contribution the period of the rhythm differs from the day length. Arti-
from starlight under clear skies. Adapted from (52); figure courtesy of T. Longcore. ficial light exposure at times that conflict with these natural
cues is an environmental effect that can interfere with this
behaviors evolved over billions of years in the presence of entrainment.
only natural sources of light at night. In addition, some species show sensitivity to the po-
larization of light (119, 120). Polarization refers to the
The scale of ALAN impacts on wildlife plane in which light waves travel. Light can become po-
larized by reflection from surfaces such as water, which
Scientists have studied at least 160 species for effects due
presents a special challenge to aquatic species near sources
to light exposure. They have observed harms at levels from
of ALAN (121, 122). The example of polarization effects
individual plants and animals all the way up to entire popu-
shows that when evaluating the impact of ALAN on wildlife,
lations (58, 59). Nearly all living things react to light. Of-
we must look at factors in addition to the intensity, spectrum,
ten these reactions negatively affect both individual organ-
duration and timing of light exposure (123).
isms and entire populations. Observed effects have been seen
Modifying the outdoor spaces at night by exposing
among birds (60–62); fishes (63–65); mammals (66–68); rep-
species to artificial light causes environmental effects. There
tiles (69–71); amphibians (72–74); insects and other inver-
are few sources of natural light in the nocturnal environ-
tebrates (75–78); and plants (79–82). Effects are seen par-
ment besides the moon and stars. This light dominated the
ticularly in aquatic environments (83) including the world’s
landscape for billions of years until the invention of electric
oceans (84, 85) to depths of hundreds of meters (86).
light. ALAN can therefore be a disadvantage to species that
Exposure to ALAN disrupts natural light intensity, timing
evolved in a world without it.
and its color characteristics (87). It increases total light inten-
The sweeping changes brought about by ALAN have
sity relative to natural levels and shifts the spectrum of ambi-
many observed effects on ecosystems (Figure 6). For in-
ent light away from its natural condition and toward shorter
stance, ALAN exposure can change the interaction between
wavelengths to which many nocturnal species are especially
predatory species and their prey (126–128). This weak-
sensitive (88, 89). Poorly timed light exposure interrupts var-
ens food webs (129, 130) and can make wildlife suscepti-
ious biological activities in plants and animals (90). These
ble to other environmental harms (131–133). Other ways
activities rely on the daily and seasonal rhythms of expo-
ALAN causes environmental harms to species are by re-
sure to light in the environment. Examples include finding
ducing options for finding food (91, 92, 134); altering how
food (91–93); the time at which certain animals first emerge
species find mates and reproduce (135–138); and interfer-
from their hiding places (94, 95); plant and animal reproduc-
ing with organisms’ abilities to orient themselves and move
tion (66, 96–98); and animal migration (99) and communica-
about (56, 62, 139–141). ALAN also alters the competition
tion (100, 101). All these effects can make it difficult for or-
for resources between species by either including species in,
ganisms to survive and reproduce; it may even influence how
or excluding them from, their habitats based on their expo-
species evolve (102, 103). This adds to other environmen-
sure tolerance (142–144).
tal pressures many species face like habitat loss and climate
IDA | State of the Science 2022 | 4
Migratory birds
Although most migrating birds navigate by sensing the
Earth’s magnetic field (160), many species also rely on light
cues in the environment. Some use these cues to ‘calibrate’
their magnetic sensitivity (161, 162). Artificial light exposure
interferes with this behavior (163).
Positive phototaxis is of particular concern for the con-
servation of migrating birds. Bright lighting in cities can be-
come a beacon to some species, drawing them away from
their migratory routes (164, 165). Fixtures emitting light ver-
tically seem to have the strongest effect (140), but even ’dark
sky friendly’ lighting attracts birds at night (166). The at-
traction to light can become lethal as it promotes collisions
between birds and windows (167).
ALAN can negatively affect the distribution of birds at
points along migratory routes where birds stop to rest and
feed (168). The presence of lit cities along those routes
causes birds to fly higher than in more rural areas (169). Very
bright installations can attract so many birds that weather
radar installations can detect them (165). This fact is now
used to measure the extent of attraction of birds to bright light
Figure 5. A cartoon representation showing how ALAN exposure can make prey
species more vulnerable to predators in the wild. In lab tests of rodents, ALAN sources on landscape scales. Researchers find that period-
interferes with signaling processes beginning in the brain’s pineal gland. This in- ically switching powerful light sources off during the night
terference apparently decreases anxiety responses, such as activity in open areas can reduce this effect by providing opportunities for birds
and behaviors like standing up on the hind legs, that could increase their visibility to
predators. Figure 1 from Russart and Nelson 2018 (124). ‘trapped’ by positive phototaxis to escape (170).
Pollinating insects
ALAN can create an effective barrier in the environ-
Ecologists have studied the role that various species play in
ment to the movement of organisms. They sometimes avoid
providing what are now called ‘ecosystem services’. These
lit areas in preference to darker ones, and ALAN can dis-
are the benefits that humans receive from the natural environ-
guise barriers that can injure or kill individuals (145, 146).
ment. An example of an ecosystem service that is critical to
It can also cause phototaxis, a condition in which organ-
human wellbeing is the pollination of food crops by insects.
isms tend to move either toward light (positive phototaxis;
Many of these insects are only active at night. Some species
e.g., 133, 147, 148) or away from light (negative phototaxis;
seem to pollinate only under conditions of dim, natural light
e.g., 149, 150). Phototaxis is a cause of injury and death
such as moonlight (171).
among both birds and insects (151–153).
ALAN appears to harm at least some nocturnal pollinator
ALAN is one of the most pressing and imminent threats
species (101, 172–174). This could reduce crop yields and
to global biodiversity (154, 155). Studies suggest clear im-
threaten food supplies in some instances (175). It may even
pacts on wildlife populations due to artificial light, even from
contribute to significant population declines among pollina-
indirect exposures (156). In particular, certain types of out-
tors that some have called the ’insect apocalypse’ (176–178).
door lighting adversely affect wildlife biology (157). In some
Researchers find effects from many types of outdoor
cases it may convey advantages to invasive species (158),
lighting, including common applications such as street light-
helping them out-compete native species. Yet biological im-
ing (179), and in at least some cases, light color may dis-
pacts of artificial light sources are still mainly referenced to
rupt nocturnal pollination (180). While some pollinators may
human vision. Our understanding of the impact of artificial
simply seek out darker places, they may find conditions there
light on species beyond our own is therefore hindered by the
less suitable (181). Further work is needed to firmly establish
convention of measuring light in reference to human vision.
importance of the threat and which lighting changes make the
Scientists stress the need to take into account the different vi-
greatest improvements for pollinators.
sual systems of animals in comparison to humans (89, 159).
ALAN is likely responsible for the death of millions of
birds and insects each year. In the following subsections, we
focus on these two classes of animals.
IDA | State of the Science 2022 | 5Figure 6. Routes by which ALAN exposure can influence interactions between different species. The figure shows some of the ecological consequences of those interactions.
Figure 7 in Gaston et al.. 2014 (125), licensed under CC-BY-3.0.
3 Human Health moonlight and 100 times less intense than the amount of light
in a bright indoor office environment. The long-term effects
Scientific evidence establishes a link between ALAN exposure of this kind of light exposure are unknown.
and adverse human health consequences. These include dis- The production of melatonin varies over the 24-hour day.
ruptions in chemical signaling in the body, certain kinds of Researchers guessed that there must be some way by which
changes at the genetic level, and shifts in sleep/wake cycles the body senses light in the environment. They suspected
set by natural light sources. These effects may contribute that it might not relate to our image-forming sense of sight.
to the incidence of certain chronic diseases in some people. In 2001, Professor George Brainard and his co-workers dis-
These conclusions are largely drawn from controlled studies covered the missing piece of the puzzle. They found evi-
of exposures to indoor lighting, suggesting caution in inter- dence for the chemical machinery in light-sensitive cells in
preting the influence of outdoor lighting on health. the retina of the eye that couples light exposure to the sys-
tem regulating the circadian rhythm (195). This machinery
involves a substance called melanopsin that is very sensitive
The light-melatonin connection
to blue light (196).
The relationship between outdoor ALAN exposure and hu- ipRGCs exposed to blue light send signals to the master
man health and wellbeing is controversial. Replicating ur- circadian “clock” in the brain. This establishes a timing ref-
ban environments and using human participants is difficult erence for other such ‘clocks’ in various organs and systems
to achieve in practice. This leads researchers to rely on of the body. Those clocks in turn govern various biological
lab studies carried out on certain animals, such as mice activities (197, 198). Exposure to ALAN can cause the mas-
and rats, which serve as well-understood models of biol- ter clock to go out of sync with the natural light pattern of
ogy in mammals generally. In these studies, ALAN ex- the 24-hour day (199). The consequences of such resets are
posure seems to have effects on the entire life cycle, from still not fully understood. And some of the peripheral clocks
childhood (182, 183) and adolescence (184, 185) to old seem to be sensitive to light on their own, independent of the
age (186, 187). In particular, these effects seem to result brain (200).
from short-wavelength (“blue”) light. While exposure to blue Further, it is now recognized that light exposure makes
light during the day is important for healthy circadian func- changes at the level of our genetic code. While it is not known
tioning (188), exposure to this light at night can disrupt the to alter our DNA, the molecule that spells out that code, light
human circadian rhythm. This can affect everything from the can cause “epigenetic” changes in humans (201, 202). These
timing of hormone release in the body to the duration and changes can switch genes “on” or “off”, altering their normal
quality of our sleep (189). The significance of these effects roles. Some of those genes relate to the function of our cir-
depend on the intensity of blue light and and the timing and cadian clocks. Epigenetic changes to those genes appear to
duration of the exposure. increase the risks of certain cancers (203), particularly breast
Exposure to light at inappropriate times during the 24- cancer (204, 205).
hour day delays or prevents the secretion of melatonin (190).
This powerful antioxidant is a hormone that interacts with The consequences of frequent ALAN exposure
the immune system (109, 191). Low-intensity artificial light
Frequent exposure to excessive light at night may be an
can suppress melatonin production (192). As little as 5 lux
emerging lifestyle risk along with other factors associated
of light can yield this effect in some particularly sensitive
with shiftwork, contributing to various health problems.
people (193, 194); that is about 50 times brighter than full
These include obesity (206–208), diabetes (209, 210), and
IDA | State of the Science 2022 | 6certain cancers (211–213) such as that of the breast (214– human health and wellbeing awaits further research. Part of
216) and prostate (217–220). ALAN exposure also seems to the challenge is telling the influence of ALAN apart from that
promote the more aggressive spread of some types of can- of other types of pollution, such as noise and air, alongside
cer (221). It can make cancer resistant to even the best avail- other environmental stressors.
able drug therapies (222) and weaken the body’s self-repair
mechanisms (223).
Some epidemiological studies find strong correlations be-
4 Public Safety
tween indications of ALAN from satellite data and the in- The belief that outdoor lighting improves traffic safety and
cidence of breast and prostate cancers, suggesting that out- discourages or prevents crime is common. It may explain in
door light exposure is an influence (224, 225). At the same part the rapid growth in the use of outdoor light at night in
time, critics point out the reliance on the use of satellite data recent years and decades. There are cases where the care-
to predict disease-related ALAN exposures (226). This may ful application of outdoor lighting may improve nighttime
make the results of some studies less reliable because satellite safety, but there is no general benefit supported by scientific
measurements are only crude estimates of the actual doses of evidence.
ALAN from outdoor sources that most people receive.
A more common way that ALAN exposure triggers ef-
Traffic safety and crime
fects in humans is by causing insomnia (227, 228). Melatonin
production and cycles of sleep and wakefulness follow each There are many conflicting research results on this topic.
other. Chronic light exposure at night associated with night Some studies find that adding lighting to outdoor spaces
shift work can cause these two cycles to decouple (229).The reduces crime and road collisions (256, 257), and even
result is often poor quality sleep and low sleep duration (230). recommend particular illumination levels based on the re-
Many social and health consequences associate with frequent sults of field experiments (258). Others find either a nega-
insomnia (231, 232), posing a threat to both public health and tive effect (259), no effect at all (260–262), or unclear re-
worker safety and productivity (233, 234). sults (263, 264). Some researchers ask whether reducing
outdoor lighting in areas prone to either crime or traffic acci-
Influences on health outcomes dents leads to poorer outcomes. Little evidence has emerged
Health practitioners now recognize the roles that light and to support this hypothesis (265).
darkness play in healing from disease and medical proce- Both traffic and crime studies are notoriously challeng-
dures. ALAN exposure delays or prevents recovery from ing to design. In particular, it is difficult to properly account
stroke (235, 236), hardening of the arteries (237), skin for all the variables that might alter the results. For exam-
wounds (238), and whole-body inflammation (239). Con- ple, a road safety study about lighting might fail to account
trolling ALAN exposures in places like hospitals results in for changing traffic volume throughout the night. Some vari-
better health outcomes (240, 241). The growth of outdoor ables may have a stronger influence on the observations than
lighting may be encouraging the spread of communicable lighting changes.
diseases (242). It may also create conditions for new and dev- Sometimes these variables are subtle effects that add up
astating diseases, such as COVID-19, to emerge (243, 244). to important results. It can be easy to assign responsibility
Other studies identify ALAN as an influence on the pro- to lighting even though it actually contributed very little. As
cess of normal aging (245). Nighttime light exposure and a result, many of the claims about outdoor lighting and its
frequent disruption of the circadian rhythm relate to mental impact on crime and traffic safety – for better or worse – may
illness (246–249), improper signaling between nerves (250), be fundamentally wrong (266, 267).
and the onset of dementia (251), and it may play a role in the Researchers have not been able to predictively model
the incidence of autism (252). Babies born to some pregnant the way outdoor lighting might affect safety and security.
women exposed to ALAN suffer from certain developmen- This is one reason why it is difficult to establish the signif-
tal defects (253, 254). On the other hand, limiting nighttime icance of lighting in studies. There is no clear known “dose-
light exposure — especially blue light — helps maintain a response” relationship that may predict appropriate lighting
normal circadian rhythm. It can ward off some abnormalities levels (268). In other words, even if lighting influences out-
that may lead to disease (255). comes, scientists can’t determine how much light is required.
We now understand much about how ALAN interacts International lighting standards often do not clearly estab-
with our health. However, our knowledge is incomplete. It is lish benchmarks for the amount of light at night that drivers
not possible now to directly connect outdoor light at night ex- and pedestrians need on the basis of scientific evidence (269).
posure to the incidence of disease in individual people. The There are only a few instances in which the issue has been
interplay between the timing and duration of ALAN expo- rigorously studied, e.g., (38), and it is unclear whether the
sure, along with the brightness and color of the light, are key results are universally applicable. Decision makers, from
factors; however, whether outdoor light pollution influences elected officials to lighting designers, often substitute their
intuition when guidance is lacking. In a belief that more of
IDA | State of the Science 2022 | 7something is always better, they often specify too much light 5 Energy Use and Climate Change
relative to actual needs.
The amount of light used in outdoor spaces at night may Wasted outdoor light at night is wasted energy. The world
not reflect public expectations for feelings of safety and com- remains highly dependent on fossil fuels to generate electric-
fort (270), and artificial light itself may influence the human ity. Since light pollution represents a waste of energy, it also
perception of fear (271). In some cases, over-lighting can contributes directly to climate change.
itself become the source of safety hazards (272). However,
properly designed lighting can reduce light pollution and save
Light and global energy demand
energy without compromising on public feelings of safety in
outdoor spaces at night (273). Electricity used to power outdoor lighting once accounted
for about 1.5% of global power consumption (283–285).
Automotive and roadway lighting Researchers hypothesized that the introduction of energy-
efficient solid-state lighting would reduce this consumption.
No one doubts that automotive lighting has clear public safety
Many governments rushed to deploy the new technology in
benefits, but this kind of lighting may itself be the source of
the past decade. As the price of SSL products declined, the
objectionable light pollution. There is little evidence to date
adoption rate increased. The motivations for this included
on the contribution of automobile lights to light pollution.
reduced cost of operation and meeting the requirements of
Some early work suggests that the impact is not small (274–
“green” policies.
276). Many expect autonomous (self-driving) vehicles to be-
At first glance, the high energy efficiency of SSL seems
come common in coming decades. Researchers are only be-
to be good for the environment. For example, the United
ginning to study what this means in terms of reducing the
Nations Environment Programme estimates that a transition
need for roadway lighting in the future (277).
to energy efficient lighting would reduce global electricity
The hazards of glare
demand for lighting by 30–40% by 2030 (286). The rapid
adoption of SSL may, however, unintentionally worsen the
Glare from bright artificial light sources is a particular con-
problem of light pollution. SSL makes outdoor light less ex-
cern for nighttime safety. It results from intense light rays
pensive and more convenient to consume. In turn, cheaper
entering the eye directly from a source. Some of that light
light may cause the use of more and brighter light at night
scatters inside the observer’s eye, reducing the contrast be-
where it is not needed.
tween foreground and background. This effect makes it dif-
ficult to see objects as distinct from what surrounds them. In The “greenwashing” of solid-state lighting
addition, the pupil of the observer’s eye contracts, reducing
As ALAN has become cheaper to produce, the world has con-
visibility by dimming the appearance of the entire scene.
sumed more of it. In fact, humans now consume thousands
Glare reduces the visibility of objects at night for mo-
of times more lumens of light than they did in the historic
torists, pedestrians and bicyclists. Although some older
past (287). There are now signs of what economists call a “re-
observers report stronger sensations of glare from certain
bound effect” in lighting. This is thought to result from the
sources, it seems to affect people of all ages (278). Some
improved energy efficiency and long lifetime of SSL prod-
modern lighting sources, like LED, can make glare worse
ucts. In such conditions, increased consumption of light at
by emitting considerable light at very shallow downward an-
night erodes away the expected savings in energy use and re-
gles (9) and also by using non-uniform light sources with in-
duction of greenhouse gas emissions. Some researchers now
sufficient optical diffusion (279).
question whether SSL is truly “sustainable” lighting (288).
The perception of glare seems to vary with the wave-
By the mid-2010s, the average country’s annual eco-
length of light involved. In general short-wavelength (’cool’)
nomic output was changing at a rate that matched that coun-
light causes stronger glare than long-wavelength (’warm’)
try’s increase in light at night consumption (1), although large
light (280). Observers report that it takes about the same
variations among countries existed. That observation sug-
amount of time to recover from glare exposure no matter the
gests that the cost savings from the switch to SSL went into
color of light (281). The severity of glare appears to relate
deploying new outdoor lighting. If true, it means that SSL
more to the ’dose’ (light intensity times exposure duration)
has not to date brought a reduction in world energy use. The
rather than to the color (280). If the background surrounding
authors of the landmark 2017 study that made these findings
a glare source is higher in luminance, its perceived intensity
wrote that their results are “inconsistent with the hypothesis
is lower. Warmer light backgrounds reduce perceived glare
of large reductions in global energy consumption for outdoor
more than cooler backgrounds (282).
lighting because of the introduction of solid-state lighting.”
Claims about the environmental benefits of SSL may be,
at best, overstated. Some researchers conclude from this that
a new definition of ‘efficiency’ is needed (9). It would con-
sider the total cost of outdoor light at night over the full life
IDA | State of the Science 2022 | 8cycle of outdoor lighting products and include factors be-
yond just the cost of electricity, such as harm to the envi-
ronment. Redefining efficiency in this way may help gov-
ernments make better decisions about outdoor lighting in the
future. It is furthermore unclear whether the root of the prob-
lem is in the technology itself or how it is applied, and hence
whether a shift in the ways in which SSL is deployed might
result in a different outcome.
The total cost of outdoor lighting Figure 7. Average exposure to light pollution in the continental United States by
racial/ethnic group. The bars show population-weighted average zenith night sky
Solid-state lighting may not provide any meaningful environ-
brightness levels in units of millicandelas per square meter. Figure 4 in Nadybal,
mental benefits in terms of reducing carbon emissions. Re- Collins and Grineski, 2020 (292).
alizing the promise of SSL requires rethinking how govern-
ments regulate outdoor lighting. Otherwise, SSL may well
higher nighttime light exposures. These conditions can add
make the problem of light pollution worse. Its impacts have
to other social and environmental stressors such as poverty
costs to the environment that can’t be measured in currency
and exposure to air and water pollution, affecting quality of
alone.
life.
The social and financial benefits of SSL seems to fade if
Other approaches link light at night exposure to specific
one considers the total environmental cost of lighting. For
health outcomes that may harm certain groups more than oth-
example, one study of a SSL retrofit program in the United
ers (293, 294). There are also limited results from established
States found a ten-year rate of return of +118.2% based
fields such as environmental psychology (295, 296). For in-
solely on savings due to reduced electricity consumption. Re-
stance, feelings of “safety” can lead people to accept lower
searchers then adjusted the return for externalities such as the
lighting levels (297). Biased perceptions may drive the puni-
social costs of poor health outcomes that may be related to
tive installation of lighting in certain neighborhoods.
ALAN exposure and the benefit of avoided carbon emissions.
Lastly, some scholars have criticized framing the idea of
The resulting rate of return dropped to –146.2% (289).
“darkness” in terms how outdoor light at night use can affect
SSL programs become less attractive when the negative
marginalized people (298, 299). They argue that failing to
consequences of ALAN are included in return-on-investment
learn from the lessons of environmental history may result
calculations. The jury remains out on the question of whether
in simply repeating mistakes of the past. Closely related to
SSL can deliver its promised environmental benefits without
this is the idea that light pollution is harmful to people whose
a reduction in outdoor light consumption.
religious or cultural practices rely on access to the night sky.
The erasure of the stars from view due to skyglow separates
6 Light and Social Justice people from this resource. Some argue that, in particular, it
threatens Indigenous traditions and knowledge systems based
We know very little about how ALAN affects people in so- on accessibility of the natural night sky (300).
cial contexts. Light at night may be used in ways that affect
neighborhoods according to the race of the people who live
in them. That may make light at night use a matter of social 7 Space Light Pollution
and environmental justice.
The number of artificial satellites surrounding the Earth is in-
We know little about the social implications of using out- creasing rapidly. Satellites reflect sunlight to the ground and
door light at night. Remote sensing of light at night from change the appearance of the night sky. Because they raise
space can show certain patterns of light use. These obser- night sky brightness, they are a new kind of light pollution
vations may reveal social inequities in other variables oth- threat.
erwise unnoticed (290). Poor social outcomes may follow Artificial satellites have orbited the Earth since the late
from the application of outdoor light. Considerations include 1950s. Until recently, they were not considered a source of
equity, health outcomes, mobility barriers, and community light pollution. That perception changed in May 2019, when
cohesion (291). The only comprehensive study to date on the launch of 60 satellites in the SpaceX “Starlink” project
this topic looked at the social aspects of lighting in the U.S. ushered in a new era in the use of outer space (301). Private
only (292). Researchers found that Americans of Asian, His- commercial space companies have since announced plans to
panic and Black descent tend to live in brighter neighbor- launch about 100,000 new satellites. They intend the satel-
hoods (Figure 7). In these areas, skyglow is about twice as lites to expand broadband internet access around the world.
high as in predominantly white neighborhoods. They further Yet, some researchers question whether satellites are neces-
note that lower socioeconomic status is also associated with sary to achieve this goal (302).
IDA | State of the Science 2022 | 9Satellites are increasingly considered an emerging form 8 Knowledge Gaps and Research
of light pollution (303–305). They impact the night sky in
two key ways. First, they reflect sunlight to the night side Needs
of Earth. Illuminated satellites appear as bright, moving
While we have learned much about the effects and costs of
points of light. They can affect activities of both amateur and
ALAN, there is also much we still do not know. Here we sum-
professional astronomers (306–309). Second, satellites can
marize key research questions in the coming decade.
Interest in ALAN among researchers in all fields has
grown by leaps and bounds (319). The average number of
scientific papers published each year has increased by over
1000% since 2000. Methods required to answer particular
questions increasingly span many different disciplines (320),
and the emergence of ’night studies’ as its own research field
prove that the subject is rapidly maturing (321).
The state of the science summarized in this report leads
to identifying important topics for future research:
The Night Sky
• What drives increasing ALAN emissions around the
world?
• How is night sky brightness changing on global scales?
• How bright is the night sky worldwide on cloudy
nights?
Figure 8. A simulated view of the night sky showing the brightness attributable to
a population of 64,000 Earth-orbiting satellites. The view is centered on the zenith,
with the horizon running around the outer edge; circles centered on the zenith mark Ecological Impacts
lines of constant elevation above the horizon at 10◦ , 20◦ , 30◦ and 60◦ . Warmer
colors indicate brighter parts of the sky. Unpublished results adapted from Bassa, • What are the sensitivity thresholds and spectral con-
Hainaut and Galadí-Enríquez, 2021 (309). tents at which different ALAN impacts occur for dif-
ferent species?
make the night sky itself brighter (Figure 8). This may be
true even when observers do not see the individual satellites. • Does skyglow in particular affect many or most plant
As a form of light pollution, it adds to the observed bright- and animal species? Does it impact entire ecosystems?
ness of the night sky along with skyglow caused by cities. • What are the long-term ecological consequences of
Researchers estimate that satellites already raise night sky light pollution?
brightness above natural light sources by ten percent (310).
• How does ALAN contribute to species population de-
It may rival the influence of “terrestrial” light pollution by
cline or extinction?
2030. Observers at high latitudes are thought to be affected
more than those in the tropics (311). • To what extent is ALAN responsible for insect popula-
Astronomers and space industry officials began consulta- tion declines?
tions soon after the first Starlink launch. Scientists suggested
reducing satellite brightnesses to reduce harm to their obser- Human Health
vations. Design changes dimmed the Starlink satellites, but
• Does exposure to ALAN in specifically outdoor spaces
they still exceed the target (312–316). Other recommenda-
affect human health in any way?
tions included limiting the altitude at which satellites may
orbit the Earth. • Does outdoor light at night entering indoor spaces af-
Recent efforts emphasized the need to engage industry fect sleep and health?
and regulators with stakeholders beyond astronomy (317). • Are the observed relationships between outdoor light
They also called for funding to study the problem more and to at night and health the result of cause and effect?
create a central clearinghouse for information (318). Ensur-
ing reasonable access to space for commercial development
is important, but we do not understand yet how to do so while
protecting the night sky from the effects of satellites.
IDA | State of the Science 2022 | 10Public Safety • Is there a particular “carrying capacity” of satellites in
• How does outdoor light at night relate to traffic safety? Low Earth Orbit?
• How does it relate to both violent crime and property • Are any satellite designs effective at reducing or elim-
crime? inating their impacts on the visibility of the night sky?
• Can we design better experiments to answer these We also consider questions and topics that span more than
questions definitively? one field of ALAN research as well as the application of that
• What are the characteristics of lights, such as intensity, research itself:
color, and other design features, that achieve desired
safety results? Synthetic Research
• How are various lighting metrics related? For example,
• How can the directionality, uniformity, controllability
can we model Sky Glow based on broad collections of
and spectral tuning of LED lighting support actual and
luminance?
perceived safety with minimally disruptive light lev-
els? • How does air pollution interface with ALAN?
• How far down can roadway, street and area lighting be • How are some measures of ALAN such as skyglow
dimmed during low-traffic times of the night in a safe specifically related to a suite of undesired outcomes
and legally defensible manner? (e.g., adverse ecological, health, or astronomical out-
comes)?
Energy Use and Climate Change
Applications of ALAN Research
• Has the ongoing global transition to solid-state lighting
had a net positive effect in terms of reducing electricity • How effective are outdoor lighting public policies at
consumption and the emission of greenhouse gases? reducing aspects of ALAN?
• What social, financial and environmental tradeoffs • What interventions besides public policy are available
have resulted from the LED lighting revolution? to mitigate the undesired consequences of ALAN?
• By how much does good lighting design lower electric • What specific economic benefits does astrotourism
power consumption? bring to communities?
• How effective are adaptive controls at reducing light at • What measurable benefits do dark sky places receive?
night use? What costs do they incur in managing their dark-sky
status?
• Can we better quantify the amount of carbon emissions
associated with outdoor lighting? • Which communities seek and obtain dark-sky designa-
tions and why?
• Which lighting technologies, design practices and poli-
cies can reduce light pollution and electricity usage to
minimum safe levels? Methodology
This report was compiled using as its main source the Artifi-
Light and Social Justice cial Light at Night Research Literature Database (ALANDB;
• How well does ALAN use match with indicators of https://alandb.darksky.org/), a database of scientific literature
public health along racial and economic lines? If citations curated by experts in different fields of ALAN re-
consistent disparities in the application of ALAN are search. We supplemented ALANDB with other online re-
found, why do they exist? sources such as Google Scholar (https://scholar.google.com/)
and PubMed (https://pubmed.ncbi.nlm.nih.gov/).
• Which public policies are effective in reducing ALAN We defined “scientific literature” as results subjected to
disparities across different communities? at least single-blind, external peer review and published in
what we believed to be reputable outlets. Both open-access
Space Light Pollution and non-open-access papers were considered. Where avail-
able, we considered post-publication metrics like citations in
• Are predictions about the contribution of satellites to
deciding which sources to use. We state caveats and short-
night sky brightness correct?
comings about sources where we know of them.
• How do night sky impacts vary according to the num- Generally we did not consider technical reports, white
bers of satellites, their orbital heights, and spatial dis- papers, theses and other sources that are sometimes collec-
tributions? tively referred to as “grey literature”. Future editions of the
report may be extended to include grey literature when there
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